If you speak Spanish, then you probably already know that it has two r sounds: the trill of carro “cart, car” and the light flap or tap of caro “expensive”. In both sounds, the tip of the tongue makes light contact with the roof of the mouth just behind the front teeth. In the trill, the tongue vibrates–at an average speed of 25 vibrations per second!–as air passes forcefully though the mouth. The flap is similar to the American English pronunciation of d as in rider. In fact, I’ve taught native Spanish speakers who misspell it as d.
You can see animations of both r types here; click on “vibrantes”.
Although both the trill and the flap seem exotic to English speakers, they are much more common world-wide than the gliding English r. This is one of the many cases where something that appears unusual about Spanish turns out to be something unusual about English. (Perhaps commenters will suggest more examples?) It’s also fairly common for languages to have two or more r sounds.
What fascinates linguists about r, in Spanish and elsewhere, is the simple fact that it has so many variations. Every other sound is defined by where and how it’s pronounced. But r can be articulated either in the front of the mouth (as in Spanish), in the back (Hebrew, French, German), or in between (English). The tongue’s movement in making the sound also varies, including trills, flap, and glides. This heterogeneity makes r a most curious critter!
For this reason, linguists generally define r—and only r—as a group of sounds that play a recognizable role in language regardless of their specific articulation. For example, any version of r can act as a buffer between a vowel and another consonant, as in both the Spanish and English versions of the phrase truco de cartas “card trick.” Members of the r group are also interconnected in language evolution, borrowing, and learning. Thus the front-of-the-mouth r of Latin real evolved into the back-of-the-mouth trill of French reel, which was borrowed into English as the mid-mouth glide of real, which interferes when English-speaking students try to pronounce the front-of-the-mouth trill of Spanish real. An r is an r is an r.